Sulfurized polyacrylonitrile and lithium-ion battery cathode active material using the same

ABSTRACT

The present disclosure relates to a sulfurized polyacrylonitrile and a lithium-ion battery cathode active material. The sulfurized polyacrylonitrile includes a structural unit. A general molecular formula of the structural unit is  C 3 HNS   n , in which n is a positive integer. The lithium-ion battery cathode active material includes sulfurized polyacrylonitrile and a sulfurized polyacrylonitrile with inserted ions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201010275623.1, filed on Sep. 8, 2010, inthe China Intellectual Property Office, the disclosure of which isincorporated herein by reference. This application is related tocommonly-assigned applications entitled, “METHOD FOR MAKING CONJUGATEDPOLYMER,” filed on Mar. 18, 2011 and application Ser. No. 13/051,123,now U.S. Pat. No. 8,273,829; “METHOD FOR MAKING SULFURIZEDPOLYACRYLONITRILE,” filed on Mar. 18, 2011 and application Ser. No.13/051,117, now U.S. Pat. No. 8,372,919; “PHOTOELECTRIC CONVERSIONCOMPONENT AND METHOD FOR MAKING THE SAME,” filed on Mar. 18, 2011 andapplication Ser. No. 13/051,148, now U.S. Pat. No. 8,450,605.

BACKGROUND

1. Technical Field

The present disclosure relates to sulfurized polyacrylonitrile andlithium-ion battery cathode active material using the same.

2. Description of Related Art

Polyacrylonitrile (PAN) is a high polymer composed of saturated carbonskeleton containing cyano groups (C≡N) on alternate carbon atoms. PANitself is not conductive but can be sulfurized to form sulfurizedpolyacrylonitrile which is conductive and chemically active.Specifically, the PAN powder and sulfur is mixed to form a mixture. Themixture is then heated, thereby forming sulfurized polyacrylonitrile.The PAN may have a cyclization reaction first during the process offorming the sulfurized polyacrylonitrile. Thus, the sulfurizedpolyacrylonitrile is a conjugated polymer having long-range π-typebonds. The sulfurized polyacrylonitrile can be used as a positivematerial of a lithium-ion battery and has a high specific capacity.

However, a cyclization degree of the sulfurized polyacrylonitrilefabricated by the above method is low, namely, the amount of theconjugated π bonds in the sulfurized polyacrylonitrile are few. Thus, anelectrical conductivity of the sulfurized polyacrylonitrile is low.

What is needed, therefore, is to provide a sulfurized polyacrylonitrilehaving high cyclization degree and a lithium-ion battery cathode activematerial using the same.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, the emphasis instead being placed upon clearlyillustrating the principles of the present embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a flow chart of one embodiment of a method for making aconjugated polymer, the conjugated polymer is used for making asulfurized polyacrylonitrile.

FIG. 2, FIG. 4, FIG. 6, FIG. 8, and FIG. 10 are Fourier Transforminfrared spectroscopy (FTIR) spectrum test graphs of embodiments of theconjugated polymers.

FIG. 3, FIG. 5, FIG. 7, FIG. 9, and FIG. 11 are ultraviolet-visibleabsorption spectrum test graphs of embodiments of the conjugatedpolymers.

FIG. 12 is a FTIR spectrum test graph of embodiments of the conjugatedpolymer and the sulfurized polyacrylonitrile formed by heating a mixturecomposed of sulfur and conjugated polymer in molar ratio of 1:4 and 1:6.

FIG. 13 is an X-ray Photoelectron Spectroscopy (XPS) test graph ofsulfur elements in one embodiment of the sulfurized polyacrylonitrilefabricated by heating the mixture composed of the sulfur and theconjugated polymer in the molar ratio of 1:4.

FIG. 14 is an XPS test graph of nitrogen elements in one embodiment ofthe sulfurized polyacrylonitrile fabricated by heating the mixturecomposed of sulfur and the conjugated polymer in the molar ratio of 1:4.

FIG. 15 is a test graph showing charge/discharge curves at 0.2 Coulomb(C) rates of one embodiment of the sulfurized polyacrylonitrile servedas the lithium battery cathode active material.

FIG. 16 is a test graph showing charge/discharge cycling curves in avoltage range from 1 volt (V) to 3.7 V of one embodiment of thesulfurized polyacrylonitrile served as the lithium battery cathodeactive material.

FIG. 17 is a test graph showing charge/discharge cycling curves in avoltage range from 1 V to 3.6 V of one embodiment of the sulfurizedpolyacrylonitrile served as the lithium battery cathode active material.

FIG. 18 is a test graph showing discharge curves under differenttemperatures of one embodiment of the sulfurized polyacrylonitrileserved as the lithium battery cathode active material.

FIG. 19 is a test graph showing discharge curves under different currentdensities of one embodiment of the polyacrylonitrile sulfide served asthe lithium battery cathode active material.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone.

First, a conjugated polymer for making a sulfurized polyacrylonitrile isprovided.

Referring to FIG. 1, one embodiment of a method for making theconjugated polymer includes the following steps:

S1, providing polyacrylonitrile (PAN), a first solvent, and a catalyst;

S2, dissolving the PAN in the first solvent to form a PAN solution, anduniformly dispersing the catalyst into the PAN solution; and

S3, heating the catalyst dispersed PAN solution to induce a cyclizingreaction of the PAN, thereby forming a first conjugated polymer solutionhaving a conjugated polymer dissolved therein.

In the step S1, the first solvent can completely dissolve the PAN. Thefirst solvent can be a polar organic solvent. The polar organic solventcan be dimethyl formamide (DMF), dimethylacetamide (DMAC), dimethylsulfoxide (DMSO), malononitrile, sulfolane, or ethyl nitrite. Amolecular weight of PAN is not limited. In one embodiment, the molecularweight of PAN is in a range from about 1000 to about 100000. Thecatalyst can be metal powder, metal oxide powder, metal salt powder,sulfur powder, or any combination thereof. Metal elements in the metalpowder, metal oxide powder, and metal salt can be transition elements,group IVA elements, group VA elements, or any combination thereof. Thecatalyst can be either soluble or insoluble to the PAN solution. Themetal powder can be silver powder, copper powder, tin powder, ironpowder, cobalt powder, or nickel powder. The metal oxide powder can betitanium oxide powder, copper oxide powder, or iron oxide powder. Themetal salt can be cobalt salt, tin salt, copper salt, nickel salt, orzinc salt, for example, cobalt nitrate (Co(NO₃)₂), zinc chloride(ZnCl₂), or antimony chloride (SbCl₃). If the catalyst is insoluble tothe PAN solution, the smaller the diameter of the catalyst particles,the easier the catalyst particles are dispersed into the PAN solution,the faster the cyclizing of the PAN, and the faster the conjugatedpolymer is formed in the step S3. The diameter of the catalyst particlescan be in a range from about 50 nanometers (nm) to about 500 micrometers(μm). In one embodiment, the diameter of the catalyst particles is in arange from about 100 nm to about 100 μm. In addition, the more catalystthat is dispersed, the faster the cyclizing of the PAN. A mass ratio ofthe catalyst to the PAN can be in a range from about 1:0.2 to about 1:6.

In the step S2, a mass concentration of the PAN solution is not limited.In one embodiment, the mass concentration of the PAN solution is in arange from about 0.1% to about 10%.

In step S3, the PAN solution with the catalyst therein can be heated ina water or oil bath. The heating temperature can be in a range fromabout 80 degrees Celsius to about 300 degrees Celsius to induce thecyclizing reaction of the PAN within a short period, thereby forming asoluble conjugated polymer dissolved in the first solvent. In addition,an insoluble conjugated polymer cannot be formed and cannot beprecipitated from the first conjugated polymer solution in the heatingtemperature range. The heating period can be set to sufficiently formsoluble conjugated polymer. The heating period can be in a range fromabout 5 minutes to about 20 minutes. Color changes of the PAN solutioncan be observed to determine if the conjugated polymer is formed or not.In one embodiment, when the PAN solution color becomes black, theconjugated polymer is formed. The darker the solution color, the higherthe conjugated polymer cyclizing level.

In the heating process, the PAN solution with the catalyst therein canbe further agitated to sufficiently mix the catalyst and the PAN in thefirst solvent, and speed up the formation of the conjugated polymer. Theagitating method is not limited, and can be magnetic stirring,ultrasonic dispersion, or mechanical agitation. An agitating rate is notlimited. If the mechanical agitation is used, the agitating speed can bein a range from about 100 rotations per minute (rpm) to about 1000 rpm.

The first conjugated polymer solution is composed of the conjugatedpolymer formed by cyclizing the PAN, the catalyst, and the firstsolvent. The conjugated polymer is soluble in the first solvent. Thecatalyst can be soluble or insoluble in the first solvent. If thecatalyst is not insoluble in the first solvent, the catalyst exists inthe form of particles.

Furthermore, after forming the first conjugated polymer solution, a stepS4 can be provided to remove the catalyst and the first solvent.

In step S4, if the catalyst is insoluble in the first solvent, thecatalyst can be filtered out of the first conjugated polymer solutionusing a filter gauze. The filter gauze defines a plurality of holeshaving diameters less than the diameters of the catalyst particles. Ifthe catalyst is soluble in the first solvent, the catalyst can beseparated out along with the first solvent. In one embodiment, a solventfilter can be used to separate out the first solvent and the catalystfrom the first conjugated polymer solution, thereby only leaving thepure conjugated polymer having a black powder shape. In anotherembodiment, the first conjugated polymer solution can be slowly heatedbelow a low temperature (e.g. 80 Celsius degrees), thereby volatilizingthe first solvent. After the first solvent is volatilized, a resonancefilter or a centrifugal machine can separate the catalyst and theconjugated polymer from each other according to specific gravitydifference or electromagnetic difference therebetween.

The method for making the conjugated polymer is described by thefollowing examples.

Example 1

In this example, the mass concentration of the PAN solution is about 5%.The catalyst is silver powder having a particle diameter of about 100μm. The silver powder and the PAN are dispersed in the PAN solution witha ratio of the silver powder to the PAN being about 1:2.5. The PANsolution with silver powder is heated at about 150 degrees Celsius by anoil bath, along with agitating the PAN solution at a rotating speed ofabout 500 rpm.

Specifically, the first solvent is DMF in the PAN solution. The PANsolution is heated for about 12 hours to cyclize the PAN until the PANsolution becomes black, thereby forming the conjugated polymer by thePAN cyclizing reaction.

Furthermore, the silver powder is separated out using the filter gauze,and the first solvent is separated out using the solvent filter, therebyforming the conjugated polymer.

Referring to FIG. 2, the achieved conjugated polymer of Example 1 istested. Wherein the characteristic absorption peak at 2242 cm⁻¹corresponds to the C≡C bonds, the characteristic absorption peak at 2938cm⁻¹ corresponds to the CH₂ bonds, the characteristic absorption peak at1387 cm⁻¹ corresponds to the CH bonds, and the characteristic absorptionpeak at 1670 cm⁻¹ corresponds to the C═N bonds or C═C bonds in theconjugated polymer. The characteristic absorption peak of C═N bonds orC═C bonds at 1670 cm⁻¹ indicating the PAN cyclizing reaction hasoccurred during the above method.

In addition, the non-conjugated unsaturated polymer can absorbultraviolet light having a relatively short wavelength. If a polymer hasconjugated typed double bonds, the polymer can absorb ultraviolet lighthaving a relatively long wavelength or visible light. The greater theconjugated typed double bonds, the more the polymer absorbs theintensity of the light having the long wavelength. Referring to FIG. 3,an ultraviolet-visible absorption spectrum test graph of the obtainedconjugated polymer of Example 1 is measured. The conjugated polymerintensively absorbs ultraviolet light having a wavelength in a rangefrom about 300 nm to about 400 nm, and weakly absorbs ultraviolet lighthaving a wavelength in a range from about 400 nm to about 600 nm andvisible light having a wavelength greater than 600 nm. Thus, theabsorption of ultraviolet light and visible light having a longwavelength indicates that the conjugated typed double bonds exist in theconjugated polymer.

It is further proven that the soluble conjugated polymer has been formedin Example 1 by analyzing FIGS. 2 and 3.

Example 2

In this example, the mass concentration of the PAN solution is about 3%.The catalyst is the sulfur powder having a particle diameter of about100 nm. A mass ratio of the sulfur powder to the PAN is about 1:0.5. ThePAN solution with the sulfur powder therein is heated at about 150degrees Celsius by an oil bath, along with agitating the PAN solution ata rotating speed of about 500 rpm.

Specifically, the first solvent is DMSO. The PAN solution is heated forabout 24 hours to cyclize the PAN until the PAN solution becomes black,thereby forming the conjugated polymer by the PAN cyclizing reaction.

Furthermore, the sulfur powder is separated out using the filter gauze,and the first solvent is separated out using the solvent filter, therebyforming the conjugated polymer.

Referring to FIG. 4, the achieved conjugated polymer of the example 2 istested, the characteristic absorption peak at about 1668 cm⁻¹corresponds to C═N bonds or C═C bonds. The characteristic absorptionpeak at about 1668 cm⁻¹ indicates a cyclizing reaction of the PAN hashappened during the above method.

Referring to FIG. 5, the conjugated polymer of Example 2 weakly absorbsthe ultraviolet light having a wavelength in a range from about 400 nmto about 600 nm and the visible light having a wavelength greater than600 nm. Thus, the absorption of the ultraviolet light or visible lighthaving a long wavelength indicates that the conjugated double bondsexist in the conjugated polymer.

It is further proven that the soluble conjugated polymer of Example 2 isformed by analyzing the FIGS. 4 and 5.

Example 3

In Example 3, the mass concentration of the PAN solution is about 1%.The catalyst is zinc chloride. A mass ratio of zinc chloride to the PANis about 2.44:1. The PAN solution with the zinc chloride dispersed isheated at about 150 Celsius degrees by an oil bath, along with agitatingthe PAN solution at a rotating speed of about 500 rpm.

Specifically, the first solvent is DMF. The PAN solution is heated forabout 24 hours to cyclize the PAN until the PAN solution becomes black,thereby forming the conjugated polymer by cyclizing reaction of PAN.

Furthermore, the zinc chloride and the first solvent are separated outusing the solvent filter, thereby achieving the purified conjugatedpolymer.

Referring to FIG. 6, the achieved conjugated polymer of Example 3 istested. The characteristic absorption peak at 1655 cm⁻¹ corresponds toC═N bonds or C═C bonds. The peak at about 1655 cm⁻¹ indicates the PANcyclizing reaction has occurred during the above method.

Referring to FIG. 7, the conjugated polymer of Example 3 uniformlyabsorbs the ultraviolet light having a wavelength in a range from about400 nm to about 600 nm and the visible light having a wavelength in arange from about 600 nm to about 800 nm. Thus, the absorption of theultraviolet light or visible light having a long wavelength indicatesthat the conjugated typed double bonds exist in the conjugated polymer,and the conjugated degree of the conjugated polymer is high.

It is proven that the soluble conjugated polymer has been formed byanalyzing the FIGS. 6 and 7.

Example 4

In Example 4, the mass concentration of the PAN solution is about 6%.The catalyst is cobalt nitrate. A mass ratio of the cobalt nitrate tothe PAN is about 27:5. The PAN solution with the cobalt nitratedispersed therein is heated at about 150 degrees Celsius by oil bath,along with agitating the PAN solution at a rotating speed of about 250rpm.

Specifically, the first solvent is DMF. The PAN solution is heated forabout 48 hours to cyclize the PAN until the PAN solution becomes black,thereby forming the conjugated polymer by the PAN cyclizing reaction.

Furthermore, the cobalt nitrate and the first solvent are separated outusing the solvent filter, thereby achieving the purified conjugatedpolymer.

Referring to FIG. 8, the achieved conjugated polymer of Example 4 istested. The characteristic absorption peak corresponding to C≡Ccompletely disappears. A group of characteristic absorption peaks near1661 cm⁻¹ corresponding to C═N bonds or C═C bonds appears. Thus, theconjugated polymer is completely cyclized.

Referring to FIG. 9, the conjugated polymer of Example 4 uniformlyabsorbs the ultraviolet light having a wavelength in a range from about400 nm to about 600 nm and the visible light having a wavelength in arange from about 600 nm to about 800 nm. The light absorbing ratio ofthe conjugated polymer is about 75%. Thus, the absorption of theultraviolet light or visible light having a long wavelength indicatesthat the conjugated double bonds exist in the conjugated polymer, andthe conjugated degree of the conjugated polymer is high.

It is proven the soluble conjugated typed polymer has been formed byanalyzing the FIGS. 8 and 9.

Example 5

In an example 5, the mass concentration of the PAN solution is about 4%.The catalyst is titanium oxide powder. A mass ratio of the titaniumoxide powder to the PAN is about 1:5. The PAN solution with the titaniumoxide powder dispersed therein is heated at about 150 degrees Celsius byan oil bath, along with agitating the PAN solution at a rotating speedof about 500 rpm.

Specifically, the first solvent is DMF. The PAN solution is heated forabout 16 days to cyclize the PAN until the PAN solution becomes black,thereby forming the conjugated polymer by the PAN cyclizing reaction.

Furthermore, the titanium oxide is separated out using the filter gauze,and the first solvent is separated out using the solvent filter, therebyachieving the purified conjugated polymer.

Referring to FIG. 10, the achieved conjugated polymer of Example 5 istested. The characteristic absorption peak at about 1589 cm⁻¹corresponds to C═N bonds or C═C bonds. The characteristic absorptionpeak at about 1589 cm⁻¹ indicates the PAN cyclizing reaction hasoccurred during the above method.

Referring to FIG. 11, the conjugated polymer in Example 5 uniformlyabsorbs the ultraviolet light having a wavelength in a range from about400 nm to about 600 nm and the visible light having a wavelength in arange from about 600 nm to about 800 nm. Thus, the absorption of theultraviolet light or visible light having a long wavelength indicatesthat the conjugated typed double bonds exist in the conjugated polymer,and the conjugated degree of the conjugated polymer is high.

It is proven the soluble conjugated polymer is formed by analyzing theFIGS. 10 and 11.

The conjugated polymer can absorb the ultraviolet light and the visiblelight, and has the conjugated typed C═C bonds or C═N bonds. Thus, theconjugated polymer has good electrical conductivity and ionicconductivity, and can be used in a lithium-ion battery. The conjugatedpolymer is soluble in some solvents to conveniently process it into afilm, thereby increasing the number of applications. A sulfurizedpolyacrylonitrile can be formed by using the conjugated polymer.

One embodiment of a method for making the sulfurized polyacrylonitrileusing any conjugated polymer of the above examples includes thefollowing steps:

M1, providing sulfur (S) or sodium thiosulfate (Na₂S₂O₃);

M2, uniformly mixing the sulfur or sodium thiosulfate with theconjugated polymer to form a mixture; and

M3, heating the mixture, thereby forming the sulfurizedpolyacrylonitrile.

In step M2, the sulfur or sodium thiosulfate and the conjugated polymercan be mixed to form a solid-solid mixture or a solid-liquid mixture. Toform the solid-liquid mixture, the sulfur or the sodium thiosulfate canbe uniformly dispersed in the above first conjugated polymer solution ofthe step S3. In another embodiment, the obtained pure conjugated polymerof the step S4 can be dissolved in a second solvent to form a secondconjugated polymer solution. The sulfur or the sodium thiosulfate isthen dispersed in the second conjugated polymer solution.

If the sulfur or the sodium thiosulfate is uniformly dispersed in thefirst conjugated polymer solution with an insoluble catalyst, thecatalyst can be separated out using the filter gauze before the step M1.If the sulfur or the sodium thiosulfate is uniformly dispersed in thefirst conjugated polymer solution with a soluble catalyst, the catalystand the first solvent can be separated out using the solvent filterafter forming the sulfurized polyacrylonitrile, thereby achieving thepurified sulfurized polyacrylonitrile. The second solvent and the firstsolvent can be the same or different. The second solvent can be DMF,DMA, DMSO, malononitrile, sulfolane, or ethyl nitrite. A molar ratio ofthe conjugated polymer to the sulfur or the sodium thiosulfate can be ina range from about 1:1 to about 1:6. If the sulfur or the sodiumthiosulfate is uniformly dispersed in the conjugated polymer solution toform the solid-liquid mixture, the mass percentage of the solute in thesolid-liquid mixture can be in a range from about 5% to about 50%. Thesolute is the sulfur or the sodium thiosulfate, and the conjugatedpolymer. Furthermore, magnetic stirring, ultrasonic dispersion, ormechanical agitation can agitate the solid-liquid mixture. If the sulfuror the sodium thiosulfate, and the conjugated polymer are mixed to formsolid-solid mixture, the solid-solid mixture can be ball-milled.

In step M3, if the sulfur or the sodium thiosulfate, and the conjugatedpolymer are mixed to form the solid-solid mixture, the heatingtemperature can be in a range from about 200 degrees Celsius to about600 degrees Celsius, the heating period can be in a range from about 5minutes to about 10 hours. If the sulfur or the sodium thiosulfate, andthe conjugated polymer are mixed to form the solid-liquid mixture, theheating temperature can be in a range from about 60 degrees Celsius toabout 150 degrees Celsius, the heating period can be in a range fromabout 5 minutes to about 10 days. The mixture can be heated in a waterbath or oil bath under an inert atmosphere. The inert atmosphere can bea nitrogen atmosphere or argon atmosphere.

In the above method, the sulfurized polyacrylonitrile is formed by avulcanization reaction of the conjugated polymer and further cyclized inthe reaction process. Thus, the sulfurized polyacrylonitrile has a highcyclization degree and conductivity.

In one example, the sulfur and the conjugated polymer are mixed in amolar ratio of about 1:4 to form a solid-solid mixture. The solid-solidmixture is ball-milled for about a half hour, and then heated at about300 degrees Celsius for about two hours in an oil bath to form thesulfurized polyacrylonitrile. In another example, the molar ratio of thesulfur to the conjugated polymer is changed to about 1:6 with the otherconditions remaining the same.

Referring to FIG. 12, curve “a” represents an FTIR spectrum of theconjugated polymer used in the Examples 1-5 for forming the sulfurizedpolyacrylonitrile. Curve “b” represents an FTIR spectrum of thesulfurized polyacrylonitrile formed in the example having the molarratio of about 1:4. Curve “c” represents an FTIR spectrum of thesulfurized polyacrylonitrile formed in the example having the molarratio of about 1:6. The characteristic absorption peak at about 2500cm⁻¹ corresponding to C≡N bonds has disappeared in the curve “b” and thecurve “c” compared with the curve “a.” The characteristic absorptionpeaks near about 1500 cm⁻¹ corresponding to C═C bonds or C═N bondsappear in the curve “b” and the curve “c,” which indicate the sulfurizedpolyacrylonitrile has been completely cyclized.

Referring to FIGS. 13 and 14, XPS test graphs of sulfur elements andnitrogen elements in the sulfurized polyacrylonitrile formed in theexample having the molar ratio of about 1:4 are shown. FIGS. 13 and 14show that reduced-state sulfur elements and oxidized-state nitrogenelements exist in the sulfurized polyacrylonitrile, which indicates thenitrogen element in cyano group (C≡N) obtains electrons, and the sulfurelement loses electrons, thereby forming N═S bonds.

The sulfurized polyacrylonitrile formed by the above method includes astructural unit. The general molecular formula of the structural unit is

C₃HNS

_(n), wherein n is a positive integer such as 1, 2, 3, and so on. Thestructural formula of the structural unit is

The structural unit can be a main portion of the molecular structure ofthe sulfurized polyacrylonitrile. The sulfurized polyacrylonitrile mayhave other structural units without being cyclized.

The sulfurized polyacrylonitrile can be used as a cathode activematerial of a lithium-ion battery. The lithium-ion battery also includesan anode active material corresponding to the cathode active material.The anode active material can be lithium metal, native graphite,pyrolysis carbon, or metal alloy. If the anode active material is anon-lithium material such as native graphite, pyrolysis carbon, andmetal alloy, then at least one of the cathode active material and theanode active material needs to have lithium ions inserted therein beforeusing.

The process of inserting the lithium ions into the sulfurizedpolyacrylonitrile at different voltages can be represented using thefollowing reaction formulas:

The sulfurized polyacrylonitrile with inserted lithium ions or only thesulfurized polyacrylonitrile without inserted lithium ions can bedirectly used as the cathode active material of a lithium-ion battery.If the sulfurized polyacrylonitrile having lithium inserted therein isused as the cathode active material, according to the above twoformulas, the cathode active material may have two possible structures.One structure of the cathode active material can include a structuralunit with a general molecular formula of

C₃HNSLi

_(n), wherein n is positive integer and can be 1, 2, 3, and so on, thestructural formula of the structural unit being

Another structure of the cathode active material can include astructural unit with a general molecular formula of

C₃HNSLi₃

_(n), wherein n is positive integer and can be 1, 2, 3, and so on, thestructural formula of the structural unit being

In one embodiment, the lithium-ion battery is fabricated by thefollowing steps:

mixing the sulfurized polyacrylonitrile, a conductive agent, and anadhesive agent to form a slurry, wherein a mass percent of thesulfurized polyacrylonitrile is in a range from about 85% to about 98%,a mass percent of the conductive agent is in a range from about 1% toabout 10%, and a mass percent of the adhesive agent is in a range fromabout 1% to about 5%;

coating the slurry on the aluminum current collector to form a cathodeelectrode;

providing an anode electrode and an electrolyte solution, wherein theanode electrode is lithium metal, the electrolyte solution is formed bydissolving the lithium hexafluorophosphate (LiPF₆) having a molarconcentration of 1 mol/L in a solvent, and the solvent is composed ofethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volumeratio of about 1:1.

The lithium-ion battery is tested for the charge/discharge properties atdifferent voltage ranges.

Referring to FIG. 15, the lithium-ion battery is cycled at a voltage ofabout 0 Volt (V) to about 3V and at about a 0.2 C rate. The chargespecific capacity of the lithium-ion battery is about 1271 mAhg⁻¹. Thedischarge specific capacity of the lithium-ion battery is about 1502.3mAhg⁻¹.

Referring to FIG. 16, the lithium-ion battery is charged to about 3.7Vat a constant current of about 0.25 mA, and then charged at the constantvoltage of about 3.7V until the current decreases to about 0 mA. Afterthat, the lithium-ion battery is discharged at a constant current ofabout 0.25 mA until the voltage drops to about 1 V. In this condition,the lithium-ion battery can be charged and discharged repeatedly foronly about three cycles.

Referring to FIG. 17, the lithium-ion battery is charged to about 3.6Vat a constant current of about 0.25 mA, and then charged at the constantvoltage of about 3.6V until the current decreases to about 0 mA. Afterthat, the lithium-ion battery is discharged at a constant current ofabout 0.25 mA until the voltage drops to about 1 V. FIG. 17 shows thatthe lithium-ion battery has a good cycling property. Thus, the cut-offvoltage for the charge of the lithium-ion battery should be set to lessthan or equal to about 3.6 V.

The lithium-ion battery is tested for the charge/discharge properties atdifferent temperatures. Referring to FIG. 18, the lithium-ion battery isdischarged at about −30° C., about −20° C., about −10° C., about 0° C.,about 10° C., about 25° C., and about 60° C. The discharge specificcapacity decreases from about 854 mAh g⁻¹ to about 632 mAh g⁻¹ as thetemperature decreases from about 60° C. to about −20° C. The dischargespecific capacity dramatically decreases from about 632 mAh g⁻¹ to about57 mAh g⁻¹ as the temperature decreases from about −20° C. to about −30°C. Thus, the lithium-ion battery can be normally operated in atemperature range from about −20° C. to about 60° C.

The lithium-ion battery is tested for the charge/discharge properties atcurrent density. Referring to FIG. 19, the lithium-ion battery isdischarged at a constant current of about 667 mAg⁻¹, about 333 mAg⁻¹,about 167 mAg⁻¹, and about 55.6 mAg⁻¹. The discharge specific capacitydecreases as the current density increases. The discharge specificcapacity is about 792 mAh g⁻¹ at the current density of about 55.6mAg⁻¹, and the discharge specific capacity is about 604 mAh g⁻¹ at thecurrent density of about 667 mA g⁻¹.

Depending on the embodiment, certain steps of the methods described maybe removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

Finally, it is to be understood that the above-described embodiments areintended to illustrate rather than limit the present disclosure.Variations may be made to the embodiments without departing from thespirit of the present disclosure as claimed. Elements associated withany of the above embodiments are envisioned to be associated with anyother embodiments. The above-described embodiments illustrate the scopeof the present disclosure but do not restrict the scope of the presentdisclosure.

What is claimed is:
 1. A sulfurized polyacrylonitrile, comprising astructural unit, wherein a general molecular formula of the structuralunit is

C₃HNS

_(n), and a general structural formula of the structural unit

wherein n is a positive integer in the general molecular formula and thegeneral structural formula.
 2. A lithium-ion battery cathode activematerial, comprising a sulfurized polyacrylonitrile comprising astructural unit, wherein a general molecular formula of the structuralunit is

C₃HNS

_(n), and a general structural formula of the structural unit is

wherein n is a positive integer in the general molecular formula and thegeneral structural formula.
 3. The lithium-ion battery cathode activematerial as claimed in claim 2, wherein a cut-off voltage for a chargeof a lithium-ion battery using the lithium-ion battery cathode activematerial is less than or equal to 3.6 V.
 4. The lithium-ion batterycathode active material as claimed in claim 2, wherein an operatingtemperature of a lithium-ion battery using the lithium-ion batterycathode active material is in a range from about −20 degrees Celsius toabout 60 degrees Celsius.